The invention relates to ultrasonic delay lines which operate in the microwave range. It is known practice to use a single-crystal medium such as a corundum bar for the fabrication of a delay line having a time delay of a few microseconds by placing an input transducer on a first end face in order to convert the electrical signal into an ultrasonic signal which propagates within the bar. The output transducer can be secured either to the opposite end face or to the same end face as the input transducer. The use of multiple reflections makes it possible to reduce the length of the bars to a considerable extent and to present the delay lines within a casing which is standardized by manufacturers of semi-conductor devices.
As disclosed in particular in French Pat. Nos. 2,239,812 and 2,241,166 filed by the present Assignee respectively on July 30, 1973 and Aug. 14, 1973, it is known that the reflecting faces of the acoustic wave propagation medium are formed in such a manner as to ensure that focusing of the wave and dispersion of the successive points of impact on the reflecting surface are carried out at the same time as reflection. By associating with this type of line a quasi-punctual transducer of the type described in French Pat. No. 2,094,443 filed on June 22, 1970 by the present Assignee, the delay lines obtained have long delay times as well as a relatively low insertion loss and are of small overall size compared with the size of single-reflection lines.
The miniaturization thus obtained helps the direct coupling between the input transducer and the output transducer, so that the output transducer provides a first direct signal having a level comprised between that of the first delayed echo and a value higher than 20 dB.